It is hard enough for people to keep up with one keychain, let alone a dozen of them for every use case in their lives.
PGP keychains allow you to have a single 24 word mnemonic seed to recover your entire digital identity, data access, etc. The UX is strictly better than the commonly suggested hodge podge of flavor of the week alternatives.
I thought everyone was "trying so hard to re-invent PGP".
> we do need a single key that can be used for all those things
We do? This is not obvious. Why does my disk encryption key need to be the same that I use to sign binaries that I release?